1. Field of the Invention
The present invention generally relates to a coiled tubing bottom hole assembly used to create an acid tunnel in a wellbore formation such that the tunnel is substantially transverse to the wellbore. In particular, the present invention relates to a coiled tubing bottom hole assembly utilizing reversible knuckle joints to create a tunnel substantially transverse to the wellbore.
2. Description of the Related Art
It has become common to stimulate a wellbore in an effort to increase the production of hydrocarbons. One method to stimulate an openhole wellbore is to create an acid tunnel that is substantially transverse to the wellbore. Acid tunneling, also referred to as chemically-enhanced drilling, is a process that uses a nozzle attached to a bottom hole assembly that is run into the wellbore with coiled tubing. Once the nozzle is located at the desired location within the wellbore, acid is pumped down the coiled tubing at a high pressure. The high pressure acid exits the nozzle and dissolves the formation adjacent to the nozzle creating a tunnel. The tunnel may be created at a specified location of the wellbore to extend beyond a damaged or non-producing portion of the well.
The bottom hole assembly preferably includes a knuckle joint used to angle the nozzle towards the side of the wellbore. The nozzle is typically located on the end of a wand connected to the knuckle joint. The diameter of the wellbore as well as the geometric configuration of the wand, nozzle, and bottom hole assembly dictate the angle at which the knuckle joint can be bent within the wellbore. The rigidity of the bottom hole assembly causes the bottom hole assembly to have a fixed radius of curvature. The radius of curvature is dictated by the length of the wand, the angle that the knuckle joint bends, and the length of the assembly from the knuckle joint to the coiled tubing connection. These dimensions define a fixed radius through which the bottom hole assembly may travel.
It is generally desired to create an acid tunnel that is substantially transverse to the wellbore so that the tunnel extends beyond a damaged area of the wellbore. It is also important that the tunnel be substantially traverse because it may be desirable to create multiple tunnels within the wellbore. It is important that the attack angle of the nozzle be sufficient to create a tunnel that is substantially transverse to the wellbore. The knuckle of the bottom hole assembly needs to position the nozzle against the wellbore to ensure that the flow of acid out of the nozzle begins to form a tunnel. If the attack angle is too shallow, the high pressure acid may simply widen the bore of the wellbore rather than creating a tunnel transverse to the wellbore. To encourage the creation of a tunnel, the knuckle joint is often configured to have a maximum bend angle of approximately fifteen degrees away from the center of the bottom hole assembly. A fifteen degree bend angle typically allows knuckle to bend causing the nozzle located on the end of the wand to come into contact with the wellbore. Typically, the knuckle will not be bent to its maximum angle until after the tunnel has begun to form. The angle required for the knuckle to contact the wellbore can be decreased by increasing the length of the wand. However, increasing the length of the wand also increases the chance that the wand may become cam locked as it traverse the wellbore and the tunnel entrance.
The coiled tubing is used to push the bottom hole assembly and increase the length of the acid tunnel. The bottom hole assembly is rigid and as discussed above, the geometry of the bottom hole assembly (i.e. the bend angle of the knuckle joint, the length of the wand, and the length from the coiled tubing to the knuckle joint) defines the radius of curvature or “build rate” of the bottom hole assembly. The build rate of the bottom hole assembly determines the “build angle” of the tunnel (i.e. how quickly the tunnel turns so that it is transverse to wellbore). Often it may be desirable to create multiple tunnels in a single wellbore. Thus, it is important to have a build rate in the tunnel that is as high as practically possible, but not so high that it exceeds the yield strength of the coiled tubing that is connected to the tunneling bottom hole assembly. For example, in a 6 inch diameter wellbore, the current bottom hole assembly for acid tunneling typically has a theoretical build rate of 300 degrees per 100 feet of tunnel. This theoretical build rate exceeds the yield radius of curvature of typical coiled tubing. It would thus be beneficial to provide a bottom hole assembly that has a lower build rate, but that also may position the nozzle against the wellbore to ensure a tunnel transverse to the wellbore is created, but with a higher initial starting angle.
Current bottom hole assemblies have been use to create acid tunnels of up to fifty feet or more in length without damaging the coiled tubing. As discussed above, the theoretical build rate of the current bottom hole assembly exceeds the elastic limit of coiled tubing. In theory, if a fifty foot tunnel is created with the maximum build rate of the current acid tunneling bottom hole assembly, then the coiled tubing would exceed yield and the force required to push the tunneling bottom hole assembly along the tunnel would exceed the buckling strength of the unsupported coiled tubing in the borehole. However, there have been instances where a fifty foot tunnel has been created without appreciable damage to the coiled tubing. One explanation for this occurrence is that the bottom hole assembly may have titled or twisted out of its original plane while creating the tunnel while at the same time creating an elongated slot that allows the bottom hole assembly to slide downwards rather than turning a corner. The bottom hole assembly most likely twisted out of plane due to the forces exerted upon the bottom hole assembly as the build rate approaches the coiled tubing's yield radius of curvature. These forces likely cause the bottom hole assembly to twist off its plane affecting the direction and location of the acid tunnel.
The twisting or tilting of the bottom hole assembly out of its original plane may cause the acid tunnel to be formed in an area other than its intended location. For example, the tunnel may not extend through the very damaged or non-producing zone as originally intended. The rotation of the bottom hole assembly may also cause the tunnel to travel substantially parallel with the wellbore rather than substantially transverse limiting the number of tunnels that may be created as well as limiting the beneficial affects from the acid tunnel.
In light of the foregoing, it would be desirable to provide a bottom hole assembly that has a reduced build rate, but still create a tunnel that is substantially transverse to the wellbore. It would further be desirable to provide a bottom hole assembly with two knuckle joints to increase the overall radius of curvature of the bottom hole assembly above the yield radius of curvature of the coiled tubing. It would be desirable to orient the two knuckle joints such that the joints would bend in the same plane. It may also be desirable to provide a bottom hole assembly with an extendable or telescopic wand to aid in the formation of an acid tunnel. It would also be desirable to provide a nozzle adapted to form an acid tunnel that encourages the bottom hole assembly to remain in its original plane as the acid tunnel is created. Moreover, it would be desirable to have the ability to adjust the angles of the knuckle joints during lateral initiation and navigation through the lateral tunnel.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.